The present invention relates to a safety device for extra-high voltage generators, particularly X-ray generators of the type comprising at least one controllable active element, such as a vacuum tube and with control grids inserted between at least one of the electrodes of the radiogenic tube and at least that of the poles of the extra-high voltage source which is intended to polarize said electrodes. These grid tubes on the one hand serve to act as switches for energizing the radiogenic tube for the exposure period when they are controlled to conduct by a positive square-wave signal applied to their control grids during exposure and on the other hand for applying between the electrodes of the radiogenic tube a voltage determined by the amplitude of said signal obtained by means of a control loop which receives a nominal value voltage preset by the user.
Generators with regulation of the radiogenic tube voltage by means of an electronic grid tube are known, particularly from the following prior art: British Pat. Nos. 689,798 and 689,799 (cf. preamble of claim 1) (U.S. Pat. No. 2,659,016 or German Patent No. 974,342), French Patent No. 1,395,015 (British Patent No. 1,077,742 or U.S. Pat. No. 3,333,104) or German Patent No. 21 16 064. In the case of the two first mentioned documents (i.e. the British and French specifications) it is preferable to use a high-voltage transformer, whose three-phase primary is star or delta connected and whose two three-phase secondaries are star-connected supplying two full-wave rectifiers, whereof one has its negative pole and the other its positive pole connected together to earth, such as, for example, described in German Patent No. 1 029 492, which supplies between its terminals an extra-high voltage (E.H.V.) symmmetrical with respect to earth.
The safety device according to the invention is intended to provide better protection for the patient, the operators and the equipment from the electrical standpoint, on the basis of the new international standards of I.E.C. 601-1. It is also intended to provide adequate radiological protection, i.e. against excessive doses of ionising radiation to the patient in accordance with international standard I.E.C. 407 and the Regulations of the Bureau of Radiological Health of the Food and Drug Administration--21 CFR Sub J 1020. Thus, during the starting of a radiogenic tube it must ensure a rapid cutting off of the E.H.V. supply in order to limit the residual energy passing through the tube to the few Joules stored in the supply cables and in order to make it unnecessary to repeat the complete examination a rapid automatic rearming makes it possible to quickly re-apply the E.H.V. to the radiogenic tube by the control of the grid tubes. This brief cutting off of the E.H.V. supply not only makes it possible to rapidly stop the radiogenic tube, whilst protecting it against high current discharges which can lead to the deposition of metal layers on the inner walls of insulating parts of its tight covering, leading to a reduction in its dielectric strength, the destruction of the filament and/or cracking of the anode, but also protects the generator and its component agaist overvoltages, overcurrents, parasitic oscillations, together with peripheral equipment of the control board and/or the computer supplying signals controlling the operation of the generator in accordance with a predetermined programme in the case of a tomo-scanner (or axial transversal tomographic apparatus assisted by a computer) and which are electrically connected or coupled to the X-ray generator.
Known safety devices used with generators of the aforementioned type are equipped with relays which react, on starting, with a relatively slow response time which can lead to the destruction of the radiogenic tube and the associated components and they interrupt the examination taking place in such a way that the irradiation of the patient has served no useful purpose.
In another generator of this type kenotrons, i.e. high-voltage vacuum diode rectifiers are inserted between the grid tubes and the radiogenic tubes in order to protect it by limiting the current, because their filaments are heated so as to limit the maximum current intensity consumed to a value close to that necessary for exposure in the case of starting the radiogenic tube. This requires two supplementary, highly insulated, heating transformers and elements for regulating the heating current of the kenotrons coupled with that of the radiogenic tube. This limitation of the anode current, without the interruption thereof, during the starting of the tube leads to an increase in the tube stoppage time and also to an increase in the recharging time of the E.H.V. cables following stoppage. This can lead to information losses, particularly in the case of an X-ray scan.